1. Field of the Invention
This invention generally relates to an apparatus for operating a gas discharge lamp, such as a fluorescent lamp, and specifically is concerned with a control circuit for operating low wattage fluorescent lamps.
2. Description of the Related Art
Gas discharge lamps, specifically common fluorescent lamps, are essentially comprised of a gas filled tube having an electrode at either end of the tube. Application of a voltage to the electrodes results in some of the gases in the tube turning to plasma and causing the lamp to luminesce. There are three basic configurations of fluorescent lamps: instantaneous start, rapid start and pre-heat. Instantaneous start lamps are lamps which are started by the application of a voltage large enough to cause the gases within the tube to instantaneously luminesce. Rapid start lamps, however, have filaments which emit electrons into the tube while a voltage is applied to the lamp and thereby assist in inducing the gases to turn to plasma and causing the lamp to luminesce. Consequently, rapid starting lamps require lower starting voltages and less deterioration of the electrodes than an instantaneous start. Finally, a pre-heat lamp is a lamp which has a glow tube or other switch which applies a voltage potential to filaments within the tube in a similar manner as rapid start lamps, however, the switch only applies the voltage when the lamp has not started thereby conserving energy.
Typically, when any gas discharge lamp is luminescing, it develops a negative resistance, once the lamp has started. The voltage required to keep the lamp operating is less than the voltage required to start the lamp. One consequence of gas discharge lamps developing negative resistances is that they draw very large amounts of current unless they are ballasted or "current limited". A gas discharge lamp is typically ballasted by placing an impedance in series with the lamp that permits the operating voltage to be applied to the lamp, but otherwise limits the amount of current that is drawn into the lamp. Both inductive coupling devices, such as chokes, transformers, resistors or capacitors are used to provide this impedance depending on operating frequency or starting voltages.
Traditional line frequency ballasts, like chokes and transformers, often are prohibitively large and will not operate on direct current. Specifically, many low wattage applications of fluorescent lamps, such as lighting in vehicles, solar powered lighting, and battery or generator-powered lighting in third world countries, necessitate that the circuit energizing the lamp be very inexpensive and very small. Unfortunately, typical ballasting circuits used in conjunction with fluorescent lamps in buildings and supplied by line voltages, e.g., 120 VAC 60 Hz, are too large and operate with alternating current only. To minimize the space and cost requirements resulting from using large ballasting elements, control circuits for fluorescent lamps, including low wattage fluorescent lamps, have been developed which supply the lamp with a high frequency alternating voltage to minimize the size of the ballasting elements needed in the circuit.
One example of such a control circuit is shown in U.S. Pat. No. 4,230,971 to Gerhard, et al., issued Oct. 28, 1980. This control circuit includes an inductive coupling element, in this case a transformer, with the lamp connected across a secondary winding of the transformer. Further, one leg of a primary winding of the transformer is connected to a DC power source and the second leg of the primary winding is connected to the collector of a switching transistor.
The base of the switching transistor is connected to a one shot multi-vibrator driven by a comparator. The comparator compares the voltage at the emitter of the transistor to a variable reference voltage. The comparator, the one shot multi-vibrator and the switching transistor generate an oscillating voltage signal as the comparator periodically causes the one shot multi-vibrator to turn the switching transistor off for a set period of time thereby causing the transformer to periodically enter a fly-back mode for that period of time. When the transformer enters the fly-back mode, an opposite voltage is generated on the secondary winding, hence, by repeatedly causing the transformer to enter the fly-back mode, the lamp receives an alternating voltage. A further feature of the control circuit shown in U.S. Pat. No. 4,230,971 is that the reference voltage supplied to the comparator can be varied. Varying the reference voltage has the effect of varying the amount of power that is supplied to the fluorescent lamp. Consequently, with the circuit configuration shown in U.S. Pat. No. 4,230,971 a dimming function for a fluorescent lamp is achieved.
One difficulty associated with control circuits of this nature is that they still require external ballasting devices to be placed in series with lamp to limit the current drawn by the lamp when the lamp is luminescing and thereby protect the lamp. While alternating the voltage applied to the lamp minimizes the current that is drawn by the lamp, the lamp still has a negative resistance which causes the current to build up very quickly. Consequently, most control circuits that supply alternating voltages to the lamps still have ballasting elements in series with the lamps. Typically, in low wattage lamp circuits, the ballasting is provided by a resistor or capacitor. Ballasts of this type often have the unfortunate effect of consuming power. This consumption of power reduces the effectiveness of the lamp in situations where the power supply has a limited capacity, e.g., a battery.
A further difficulty with low wattage circuits providing an alternating voltage to the lamp is that they usually use either a fixed oscillator or a comparator-multi-vibrator circuit in conjunction with the inductive coupling element to provide the alternating voltage signal to the lamp. With this type of circuit, however, if there is a decrease in the supply voltage provided to the circuit, there is often a corresponding decrease in the voltage applied to the lamp which results in the lamp dimming or flickering.
Further, many low wattage lamps currently available have capacitances in parallel with the lamp. For example, most pre-heat lamps have a glow tube switch and an arc and noise suppression capacitor in parallel with the glow tube. The existence of these parallel capacitances necessitates the application of higher amplitude voltages to start the lamp when high frequency voltage signals are being used to energize a low wattage lamp of this type, as the parallel capacitances oppose the alternating changes in voltage and reduce the amount of power that is transmitted to the lamp. Using a fixed oscillator or a one-shot multi-vibrator to generate the voltage signal results in a fixed amount of energy being transmitted to the lamp. Hence, control circuits providing high frequency voltage signals to low wattage lamps of this type must continuously provide a sufficiently high voltage signal required to overcome these capacitances and start the lamp even after the lamp is operating. Since the lamp requires less energy to operate than it does to start, control circuits of this type are inefficient as they continuously provide the higher starting energy to the lamp and thus unnecessarily consume energy. In applications using low wattage lamps, this problem is accentuated as the power source is often a battery having a limited capacity for providing energy.
An additional problem with the above-described control circuits for low wattage lamps is that they typically do not incorporate any protection for the circuit components from voltages resulting from fault conditions. Specifically, if a lamp is removed from a typical circuit while the circuit is energized, a large voltage, that would otherwise be absorbed by the lamp, often results when the inductive coupling device enters the fly-back mode which could potentially damage the components of the circuit. Since the circuit that induces the inductive coupling element to enter the fly-back mode typically operates at a fixed frequency, there is no way to limit or clamp the amount of energy that is stored in the inductive coupling element to a safe level.
Hence, a need therefore exists in the prior art for a control circuit for low wattage gas discharge lamps that provides a high frequency alternating voltage to the lamp which does not require any additional external ballasting elements and can either increase or decrease the amount of energy provided to the lamp depending upon the condition of the lamp and supply voltage. To this end, there is a need in the prior art for an inexpensive control circuit which uses closed-loop feedback to control the amount of current that is being drawn by the gas discharge lamp when the lamp is operating to thereby eliminate the need for external ballasting. This control circuit should also be able to determine when the lamp is not being provided sufficient energy to start and can then increase the amount of energy provided to the lamp. Further, this control circuit should also be able to detect fault conditions where the resulting voltage in the circuit reach potentially damaging levels and can then decrease the amount of energy and current being produced by the circuit to thereby protect circuit components.